This invention relates generally to magnetic recording media, and more particularly to thermally stable high density media.
Conventional magnetic recording media, such as the magnetic recording disks in hard disk drives, typically use a granular ferromagnetic layer, such as a sputter-deposited cobalt-platinum (CoPt) alloy, as the recording medium. Each magnetized domain in the magnetic recording layer is comprised of many small magnetic grains. The transitions between magnetized domains represent the xe2x80x9cbitsxe2x80x9d of the recorded data. IBM""s U.S. Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventional rigid disk.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (the magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (Hc) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hc. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the magnetization in the layer will become more susceptible to thermal decay. This decay has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the magnetic layer and V is the volume of the magnetic grain. As the magnetic layer thickness is decreased, V decreases. Thus, if the magnetic layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions.
One approach to the solution of this problem is to move to a higher anisotropy material (higher Ku). However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too large to be written by a conventional recording head. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
IBM""s co-pending application Ser. No. 09/416,364, filed Oct. 8, 1999, now U.S. Pat. No. 6,280,813, addresses the thermal stability problem by replacing the conventional single magnetic recording layer with two ferromagnetic films that are antiferromagnetically coupled together across a nonferromagnetic spacer film. Because the magnetic moments of the two antiferromagnetically-coupled films are oriented anti parallel, the net remanent magnetization-thickness product (Mrt) of this recording layer is the difference in the Mrt values of the two ferromagnetic films. However, this reduction in Mrt is accomplished without a reduction in the thermal stability (represented by KuV) of the recording layer because the volume V of the grains in each of the two separate antiferromagnetically-coupled films add constructively. While this approach appears promising, it introduces a new set of unknowns relating to the magnetic and recording/reproducing properties of this unconventional recording layer.
What is needed is magnetic recording media that will support very high density recording while retaining good thermal stability, yet takes advantage of the well-known magnetic and recording/reproducing properties of the conventional single layer granular Co alloy magnetic material.
The invention is a magnetic recording disk wherein the magnetic recording layer is formed on a special multilayered xe2x80x9chostxe2x80x9d layer. The host layer is a xe2x80x9csynthetic antiferromagnetxe2x80x9d, i.e., at least two ferromagnetic films that are exchange-coupled antiferromagnetically (AF) to one another across a nonferromagnetic spacer film so that their magnetic moments are oriented anti parallel.
In the preferred embodiment, the thicknesses and materials of the two ferromagnetic films are chosen so that the moments from the individual ferromagnetic films essentially cancel. Thus the host layer has no net magnetic moment, or a very small nonzero moment, so that it does not contribute to the Mrt of the magnetic recording layer.
The magnetic recording layer has a different composition from the top ferromagnetic film in the host layer and is ferromagnetically coupled to the top ferromagnetic film of the host layer. The magnetic volume V of the composite structure (magnetic recording layer and host layer) that determines the thermal stability will be approximately the sum of the volumes of the grains in the magnetic recording layer and the AF-coupled ferromagnetic films of the host layer. However, the magnetic moment of the composite structure is just the moment from the magnetic recording layer because the host layer is designed to have essentially no net magnetic moment. Thus the antiferromagnetic coupling between the two ferromagnetic films of the host layer provides a mechanism to increase the effective thickness of the composite structure without increasing the net Mrt value of the composite structure.
In an alternative embodiment the two AF-coupled films of the host layer have magnetic moments that are still oriented anti parallel but are deliberately different in magnitude so that the host layer has a net magnetic moment. This may be done to optimize recording performance, reduce thermal decay or design the media to certain values of magnetic moment and coercivity without changing the manufacturing process.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.